1. Field of the Invention
The invention relates to a magnetic resonance imaging apparatus, comprising an RF coil system connectable to an RF transmitter and/or an RF receiver.
2. Prior Art
It is known that upon reception of spin resonance signals the signal-to-noise ratio deteriorates as the magnetic flux density of the steady, uniform magnetic field to which the examination zone is exposed during magnetic resonance imaging decreases. In order to achieve an acceptable signal-to-noise ratio in magnetic resonance imaging apparatus with a low magnetic flux density, it is important that the RF coil system detecting the spin resonance signals has a high Q and a high sensitivity. It is known that a so-called solenoid coil comprising an adequate number of turns has such properties. However, the number of turns may only be chosen to be so large that the length of the conductor used to wind the coil amounts to less than one quarter wavelength of the operating frequency, and is preferably small in comparison therewith.
To this end, Murton and Neale disclose a coil system in which two coils, comprising three turns each are connected in parallel. Because of inevitable coil capacitances, however, the self-resonant frequency thereof is still comparatively low; this may give rise to wave propagation effects, causing increased dielectric losses and hence a decreased Q.
It is an object of the present invention to provide an RF coil system for a magnetic resonance imaging apparatus which has an improved signal-to-noise ratio and a relatively high Q factor, even in the case of comparatively low resonance frequencies. This object is achieved in accordance with the invention in that the RF coil system comprises a plurality of resonators which are tuned to the same frequency, each resonator comprising a conductor loop which consists of one or more parts and whose ends are capacitively coupled to one another, resonators which are not conductively connected to one another being inductively coupled to one another, one of the resonators being connectable to the RF transmitter or the RF receiver.
The invention thus utilizes a number of resonators, each of which is formed by a conductor loop, said resonators generally being tuned to the same frequency by means of an additional capacitor. The individual resonators are not conductively connected to one another, but are inductively coupled to one another. Only one of the resonators is connected to the RF transmitter or the RF receiver during operation. Consequently, as regards wave propagation effects and the associated dielectric losses such an RF coil system behaves as an RF coil comprising only a single turn; however, in as far as sensitivity is concerned it acts as a solenoid coil whose number of turns corresponds to the number of resonators.
An RF coil system in acordance with the invention has as many modes of oscillation as there are resonators. In one mode of oscillation the currents flow in the same direction in all resonators. This mode of oscillation relates to the lowest resonance frequency which is lower than the frequency to which the individual resonators are tuned. In another mode of oscillation, the currents in spatially neighbouring resonators always flow in opposite directions. In this mode of oscillation the highest resonance frequency is obtained, which frequency exceeds the frequency whereto the resonators are tuned. This mode of oscillation can be activated only by applying a current having the relevant frequency to one of the resonators; however, it cannot be initiated by an external uniform RF field. Therefore, when an RF coil system in accordance with the invention is used for receiving spin resonance signals it can be completely uncoupled from a transmitter coil operating at the same resonance frequency.
In one embodiment in accordance with the invention a uniform, steady magnetic field acts perpendicularly on the examination zone enclosed by the RF coil system in the operating condition. A patient is usually examined in the recumbent position in a magnetic resonance imaging tomography apparatus. The RF coil system then has a horizontally extending longitudinal axis and generates an RF magnetic field which extends in this direction, the vertical, uniform and steady magnetic field extending perpendicularly thereto in the prescribed manner.